There is a need to determine when a train vehicle operative in an automated vehicle control system is moving at zero speed or at less than a predetermined zero equivalent speed, with a fail-safe or vital zero speed indication signal being provided as desired for train operation control purposes when the train vehicle is moving at less than such predetermined zero speed. A propulsion enable signal is provided when the actual speed of the train vehicle is less than the desired speed for that vehicle. When the actual speed of the vehicle is greater than the desired speed the propulsion enable signal is not provided and the full service brake will be applied. In addition when the train vehicle is detected to be positioned adjacent to a station platform, a door open enable signal is provided when the vehicle is sensed to be moving at less than a predetermined zero equivalent speed such as 0.1 mile per hour. The provision of the propulsion enable signal and the provision of the door open enable signal has to be in a substantially fail-safe manner.
The train vehicle has to be out of a full service brake condition of operation to move the train in response to a desired speed signal. In normal operation the train can be operating along a track divided into signaling blocks of respective predetermined lengths, with a very low impedance connection being made between the track rails at the ends of each such signal block. A signal transmitter is operative with one end of each signal block at one of several frequencies and a cooperative signal receiver is coupled with the other end of each signal block for controlling the operation of a train vehicle positioned within that signal block, such as described in U.S. Pat. No. Re. 27,472 and U.S. Pat. No. 3,532,877 of G. M. Thorne-Booth and in U.S. Pat. No. 3,593,022 of G. M. Thorne-Booth et al. A published article entitled "Automatic Train Control Concepts Are Implemented By Modern Equipment" by R. C. Hoyler in the September 1972 Westinghouse Engineer at pages 145 to 151 includes a disclosure of this operation.
A signal receiver carried by the train vehicle senses a desired speed coded signal from the signal block occupied by that vehicle, which desired speed signal the train decodes and provides a desired speed command signal to the propulsion control apparatus of the train vehicle to result in energizing the propulsion motors for regulating the actual speed to correspond with the desired speed of operation along the track and within a particular signal block. The actual speed of the vehicle is obtained from a pair of tachometers operative with the wheels of the vehicle as disclosed in U.S. Pat. No. 3,810,681 of T. C. Matty. If the actual speed of the train vehicle is too low, more propulsion effort for the vehicle is required and if the actual speed is too high then braking of the vehicle is provided.